Safeties are relied on to covering the pass first, but then attacking the LOS if they read run delivering big hits.
NFL teams are looking for safeties with that combination of size, speed, strength and explosiveness. In terms of size, most NFL safeties are at least 6'1'' and weigh between 210-225 pounds. In terms of speed most safeties run in the 4.5 forty range.
A good NFL safety will also have to have jumping ability, fluid hips, good footwork in a back pedal, be very physical, good change of direction, good hands and great awareness. They also need to be able to be good in man and zone coverages. Safeties need to really time their blitzes well without giving it away to the quarterback. They also need to keep everything in front of them, reading the quarterback's eyes leading them to the football.
Here are my Top 10 Safeties for the 2011 NFL Draft.